Resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical resistance variable memory element formed using a chalcogenide glass backbone is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.
In a typical chalcogenide glass resistance variable memory element, a conductive material, such as silver, is incorporated into the chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide variable resistance element is normally in a higher resistance state. A write operation programs the element to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conductive pathway. The element may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define two logic states.
The programmed lower resistance state of a chalcogenide variable resistance element can remain intact for a considerable period of time, typically ranging from hours to weeks, after the voltage potentials are removed. The element can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such an element can function as a variable resistance non-volatile memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
A plurality of resistance variable memory devices can be included in a memory array. In doing so, it is desirable to provide a greater number of memory elements within a particular area.